Magnetic head-based systems have been widely accepted in the computer industry as a cost-effective form of data storage. In a magnetic disk drive system, a magnetic recording medium in the form of a disk rotates at high speed while a magnetic head “flies” slightly above the surface of the rotating disk. The magnetic disk is rotated by means of a spindle drive motor. The magnetic head is attached to or formed integrally with a “slider” which is suspended over the disk by a suspension assembly which in turn is attached to an actuator arm. As the magnetic disk rotates at operating speed, the moving air generated by the rotating disk in conjunction with the physical design of the slider lifts the magnetic head, allowing it to glide or “fly” slightly above and over the disk surface on a cushion of air, referred to as an air bearing. The flying height of the magnetic head over the disk surface is typically only a few tens of nanometers or less and is primarily a function of disk rotation, the aerodynamic properties of the slider assembly and the force exerted by the spring-loaded actuator arm.
Magnetoresistive (MR) sensors are particularly useful as read elements in magnetic heads, used in the data storage industry for high data recording densities. Two examples of MR materials used in the storage industry are anisotropic magnetoresistive (AMR) and giant magnetoresistive (GMR). MR and GMR sensors are deposited as small and thin multi-layered sheet resistors on a structural substrate. The sheet resistors can be coupled to external devices by contact to metal pads which are electrically connected to the sheet resistors. MR sensors provide a high output signal which is not directly related to the head velocity as in the case of inductive read heads.
Another type of magnetic device currently under development is a magnetic tunnel junction (MTJ) device. The MTJ device has potential applications as a memory cell and as a magnetic field sensor. The MTJ device comprises two ferromagnetic layers separated by a thin, electrically insulating, tunnel barrier layer. The tunnel barrier layer is sufficiently thin that quantum-mechanical tunneling of charge carriers occurs between the ferromagnetic layers. The tunneling process is electron spin dependent, which means that the tunneling current across the junction depends on the spin-dependent electronic properties of the ferromagnetic materials and is a function of the relative orientation of the magnetizations of the two ferromagnetic layers. In the MTJ sensor, one ferromagnetic layer has its magnetization fixed, or pinned, and the other ferromagnetic layer has its magnetization free to rotate in response to an external magnetic field from the recording medium (the signal field). When an electric potential is applied between the two ferromagnetic layers, the sensor resistance is a function of the tunneling current across the insulating layer between the ferromagnetic layers. Since the tunneling current that flows perpendicularly through the tunnel barrier layer depends on the relative magnetization directions of the two ferromagnetic layers, recorded data can be read from a magnetic medium because the signal field causes a change of direction of magnetization of the free layer, which in turn causes a change in resistance of the MTJ sensor and a corresponding change in the sensed current or voltage. U.S. Pat. No. 5,650,958 granted to Gallagher et al., incorporated in its entirety herein by reference, discloses an MTJ sensor operating on the basis of the magnetic tunnel junction effect.
To achieve the high areal densities required by the data storage industry, the sensors are made with commensurately small dimensions. The smaller the dimensions, the more sensitive the thin sheet resistors become to damage from spurious current or voltage spike.
A major problem that is encountered during manufacturing, handling and use of MR sheet resistors as magnetic sensors is the buildup of electrostatic charges on the various elements of a head or other objects which come into contact with the sensors, particularly sensors of the thin film type, and the accompanying spurious discharge of the static electricity thus generated. Static charges may be externally produced and accumulate on instruments used by persons performing head manufacturing or testing function. These static charges may be discharged through the head causing excessive heating of the sensitive sensors which result in physical or magnetic damage to the sensors. This phenomenon is generally known as electrostatic discharge. A discharge of only a few volts can destroy or severely damage the MR sensor. Such a discharge can occur by contact with or close proximity to a person, plastic involved in the fabrication, or components of a magnetic medium drive.
As described above, when an MR head is exposed to voltage or current inputs which are larger than that intended under normal operating conditions, the sensor and other parts of the head may be damaged. This sensitivity to electrical damage is particularly severe for MR read sensors because of their relatively small physical size. For example, an MR sensor used for high recording densities for a magnetic disk drive (on the order of 100 Gbytes/in2 or greater) is patterned as resistive sheets of MR and accompanying materials, and has a combined thickness for the sensor sheets on the order of 400 Angstroms (Å) with a width and height both on the order of 100 nm with the length and thickness of the MR sensor exposed at the air bearing surface of the MR head, while the height is buried in the body of the head. Discharge currents of tens of milliamps through such a small resistor can cause severe damage or complete destruction of the MR sensor. The nature of the damage which may be experienced by an MR sensor varies significantly, including complete destruction of the sensor via melting and evaporation, oxidation of materials at the air bearing surface (ABS), generation of shorts via electrical breakdown, and milder forms of magnetic or physical damage in which the head performance may be degraded. Short time current or voltage pulses which cause extensive physical damage to a sensor are termed electrostatic discharge (ESD) pulses. Short time pulses which do not result in noticeable physical damage (resistance changes), but which alter the magnetic response or stability of the sensors due to excessive heating are termed electrical overstress (EOS) pulses.
Several methods for reduction of ESD damage are detailed in U.S. Pat. No. 6,400,534 (resistive shunt), U.S. Pat. No. 5,757,590 (fusible links), U.S. Pat. No. 5,759,428 (laser cutting of a metal short), U.S. Pat. No. 5,748,412 (shunting with anti parallel diode pair), U.S. Pat. No. 5,644,454 (short on the suspension), etc. While each of these methods can provide certain ESD protection, factors such as cost, effectiveness in terms of variations in ESD transients, extent of protection in slider fabrication, added complexity to manufacturing, etc. have precluded their eventual implementation.
In the diode approach, for example, a pair of diodes is connected in parallel across the MR sensor, each diode pointing the opposite forward bias direction, (crossed diodes) to protect the MR device. The diode pair is intended to remain in parallel with the MR sensor during normal operation of the disk drive. Such an ESD protection scheme can lead to considerations in cost, space on the HGA, and performance issues with the MR sensor.
Electrically shorting out the MR sensor, by shorting the two ends of the sensor which connect to external devices, provides the best possible ESD protection. For example, an MR sensor is typically positioned between a pair of thin film gap layers which are in turn sandwiched between a pair of thin film shield layers. A pair of thin film leads, which are employed for transmitting the sense current through the MR sensor, also lie between the gap layers. The leads terminate at a pair of pads which are exposed for connection to drive electronics. A convenient way of protecting the MR sensor from ESD is to interconnect the pads with a thin film conductive line on the exterior surface of the MR head. This shorts the MR circuit, bypassing potential damaging current from electrical discharge. One of the best times to form the conductive line between the pads during assembly of a magnetic disk drive is at the row level which will be explained hereinafter. The problem with this technique is that the head is no longer functional while the short is applied. The short needs to be removed for testing purposes several times during the manufacturing and assembly of a magnetic hard disk drive; at row level, slider level, head-gimbal assembly (HGA) level and at head-stack assembly (HSA) level. Once the short is removed, for testing or use, the sensors are no longer protected.
Thus, in all of the known methods for providing ESD protection, one common drawback remains. None of these methods can provide the flexibility and effectiveness in ESD protection for the magnetic sensor as often as desired.
A need therefore exists for providing ESD protection that can be applied in a repeatable manner so that the head can be shorted when necessary, and the short severed when desired, such as for testing.